This invention relates to a device for detecting spatial variation in the intensity of electromagnetic radiation.
A quadrant detector that comprises four photodiodes, fabricated in a single die and equiangularly distributed about a central axis, may be used for positioning an object relative to a light source. The quadrant detector is mounted on the object so when the object is in the desired orientation, the light source is imaged on the detector and the four diodes provide equal currents. If the orientation of the object changes, at least two of the photocurrents will be different from each other and the resulting error signal may be used in a feedback loop to adjust the orientation of the object in a manner such as to achieve equality of the four photocurrents. Conventional quadrant detectors of this kind are not suitable for positioning an object relative to a faint light source. For example, such a quadrant detector could not readily be used to position a spacecraft relative to a star. The reason for this is that the energy flux received by the detector is so small that the photocurrents provided by the four diodes are subject to substantial and essentially random variations in relative magnitude, and therefore the instantaneous value of an error signal is not a reliable indication of the error in orientation of the spacecraft.
It is known to use an imaging charge-coupled device (CCD) to generate an electrical signal representative of the intensity with which light is incident on the device. One form of imaging CCD comprises a silicon die that has been processed using conventional MOS technology to form a buried channel beneath its front surface (the surface through which the die is processed). Charges are generated in the die photoelectrically. Thus, if photons enter the die, conduction electrons may be generated and these conduction electrons may enter the buried channel. By controlling clock signals that are applied to a gate electrode structure of the CCD, the charge that enters the buried channel may be confined in the channel for a substantial period. Accumulated charge may be extracted from the buried channel by application of suitable clock signals to the gate electrode structure.